[5.5] Revert Collection::sortBy to 5.4 behaviour

[themsaid]

That's how the method used to work in 5.4:
https://github.com/laravel/framework/blob/5.4/src/Illuminate/Support/Collection.php#L1259

[elynnaie]

@themsaid @taylorotwell You seem to be going back and forth a lot on this stable sort thing. Is there a plan to settle on it one way or another? I need to know if I should be able to count on having stable sorting in collections or if I need to continue to roll my own solution for the foreseeable future.